Toner for developing static charge image and method for preparation thereof

ABSTRACT

A toner for development of electrostatic images, comprising colored particles containing at least a binder resin, a colorant and a softening agent, wherein the softening agent is an organic compound having a molecular weight of at least 1,000, a solubility of at least 5 g in 100 g of styrene as measured at 25° C., and an acid value of at most 10 mg KOH/g. The toner is excellent in fixing ability, shelf stability and flowability and permits forming images having high image quality. A production process thereof is also provided.

TECHNICAL FIELD

The present invention relates to a toner for development ofelectrostatic images for developing electrostatic latent images formedon a photosensitive body by an electrophotographic process,electrostatic recording process or the like and a production processthereof, and more particularly to a toner for development ofelectrostatic images, which is excellent in fixing ability, shelfstability and flowability and capable of forming high-quality images,and a production process thereof.

BACKGROUND ART

In an image forming apparatus such as an electrophotographic apparatusor electrostatic recording apparatus, exposure to a light pattern isconducted on a photosensitive member uniformly and evenly charged toform an electrostatic latent image (electrostatic image), and adeveloper is applied to the exposed region or unexposed region on thephotosensitive member to conduct development. The developer image formedon the photosensitive member is generally transferred to a transfermedium such as paper or OHP sheet, and then fixed to the transfer mediumby a method such as heating, pressing or use of solvent vapor.

As the developer, is used a toner for development of electrostaticimages composed of colored particles comprising a binder resin in whicha colorant and other additives (for example, a charge control agent, aparting agent, etc.) have been dispersed.

As toners for development of electrostatic images, ground tonersobtained by melting and mixing a colorant and other additives in athermoplastic resin to prepare a resin composition and then grinding andclassifying the resin composition have heretofore been used mainly. Inrecent years, polymerized toners which are easy to control theirparticle diameter and permit omitting complicated steps such as grindingand classification and providing high-quality images have come to bewidely used.

In general, a polymerized toner is produced by pouring a polymerizablemonomer composition containing a polymerizable monomer, a colorant, acharge control agent, a parting agent and the like in an aqueousdispersion medium containing a dispersion stabilizer to disperse it inthe aqueous medium by means of a mixer having high shearing force,thereby forming fine droplets of the monomer composition, and thensubjecting the dispersion containing the fine droplets to suspensionpolymerization with a polymerization initiator. A polymer formed by thepolymerization of the polymerizable monomer becomes a binder resin, andthe colorant and other additives are dispersed therein.

The toner for development of electrostatic images has been required topermit forming a high-definition and high-density image having excellentimage quality, undergo no deterioration of image quality even by changesin environments such as temperature and humidity and make it possible toconduct continuous printing or copying. In addition to theserequirements, the toner for development of electrostatic images has beenrecently required to permit contributing to energy saving and copingwith the speeding-up of printing or copying and the formation offull-color images. Therefore, the toner for development of electrostaticimages has been required to improve its fixing properties such aslowering of fixing temperature without impairing the shelf stability(blocking resistance) while retaining the achievement of high imagequality.

Specifically, in image forming apparatus such as copying machines,printers and the like of the electrophotographic system, in which thetoner for development of electrostatic images is used, it has beenrecently attempted to reduce demand power. A step in which energy isparticularly consumed in the electrophotographic system is a fixing stepfor fixing a developer image (toner image) after transferring thedeveloper image on a photosensitive member to a transfer medium such aspaper. In the fixing step, a fixing roll or fixing belt heated to a hightemperature of at least 150° C. is used to fix the toner image to thetransfer medium, and electricity is used as an energy source therefor.There is a demand for lowering this fixing temperature from theviewpoint of energy saving.

There has recently been a demand for the speeding-up of copying andprinting. In particular, the speeding-up of copying and printing hasbeen more and more required with the advancement of the combination ofimage forming apparatus and the formation of personal computer network.Therefore, it is necessary to shorten the fixing time in a high-speedprinter or copying machine.

As a method for meeting such requirements from the image formingapparatus in the design of a toner for development of electrostaticimages, there is a method in which a glass transition temperature of abinder resin is lowered. When the glass transition temperature of thebinder resin is lowed, however, the resulting toner becomes poor in theso-called shelf stability because particles of the toner undergoblocking during storage of the toner or in a toner box of an imageforming apparatus, to aggregate.

More recently, color-printing and color-copying techniques have beendeveloped. In order to conduct color printing or color copying, anelectrostatic latent image on a photosensitive member is developed withcolor toners of 3 or 4 different colors to transfer the resulting tonerimage to a transfer medium at a time or successively, and the tonerimage is then fixed. Therefore, the thickness of the toner layer to befixed becomes thicker compared with a black-and-white image. In order todevelop a desired color tone by color mixing, the respective colortoners overlapped are required to be uniformly melted upon fixing ofsuch color toners.

Therefore, the melt viscosity of each toner at about the fixingtemperature thereof must be designed low compared with the conventionaltoners so as to become easy to melt. Means for lowering the meltviscosity of the toner include, for example, methods in which themolecular weight of a binder resin used is made lower compared with theresins for the conventional toners, and in which the glass transitiontemperature thereof is lowered. In any of these methods, however, thetoner becomes poor in shelf stability because the toner tends to undergoblocking.

As described above, when the methods for improving a toner so as to copewith the energy saving, the speeding-up of printing and copying and theformation of color images are adopted, the shelf stability of the toneris deteriorated. More specifically, there is an adverse correlationbetween these methods and the shelf stability.

In order to provide a toner for development of electrostatic imageshaving good low-temperature fixing ability, there has heretofore beenproposed a method in which a low-softening point substance havingparting property, such as paraffin wax, is caused to exist in a toner tolower the softening point of the toner (Japanese Patent ApplicationLaid-Open Nos. 173067/1988 and 161144/1994). However, such a toner isdifficult to achieve high image quality and balance the low-temperaturefixing ability with the shelf stability at a high level.

Specifically, Japanese Patent Application Laid-Open No. 173067/1988 hasproposed a production process of a polymerized toner, comprising thesteps of adding polyolefin wax into a monomer mixture containing apolymerized monomer and a colorant, heating the resultant mixture to atemperature higher than a polymerization temperature to dissolve thepolyolefin wax in the polymerizable monomer and then cooling the mixturedown to a temperature equal to the polymerization temperature to depositthe polyolefin wax. According to this production process, however, thepolyolefin wax is dissolved in the polymerizable monomer at the hightemperature, and a polymerization initiator is then poured therein atthe polymerization temperature, so that the control of thepolymerization reaction is difficult to fail to easily obtain a uniformtoner.

Japanese Patent Application Laid-Open No. 161144/1994 has proposed atoner in which a small amount of paraffin wax having no compatibilitywith a binder resin is contained in the resin. However, this toner islimited to the ground toner produced by mixing and kneading a binderresin, a colorant, wax and other additives with one another. andgrinding and classifying the kneaded product. In addition, such a tonercannot be expected to have sufficient low-temperature fixing ability.

Japanese Patent Application Laid-Open No. 197193/1993 has proposed apolymerized toner of a phase-separation structure that toner particlescomprise a high-softening resin (A) and a low-softening point substance(B), an A phase composed mainly of the high-softening resin is presentin the vicinity of the surface of each particle, and a B phase composedmainly of the low-softening point substance is not present in thevicinity of the surface.

However, this toner of the phase-separation structure is good inblocking resistance, but yet high in fixing temperature and insufficientin low-temperature fixing ability. It is also difficult to contain agreat amount of the low-softening point substance such as insoluble waxin a polymerizable monomer. In addition, when the low-softening pointsubstance is contained in an adding amount shown in Examples of thispublication in the toner, such a toner becomes too glossy and isdifficult to achieve good image quality.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a toner fordevelopment of electrostatic images, which has a low fixing temperature,can meet energy saving, the speeding-up of printing and copying, theformation of full-color images, and the like, has excellent shelfstability and flowability and permits forming images high in resolutionand good in image quality.

The present inventors have carried out an extensive investigation with aview toward achieving the above object. As a result, it has been foundthat the object can be achieved by containing an organic compound havinga molecular weight of at-least 1,000, a solubility of at least 5 g in100 g of styrene as measured at 25° C. and an acid value of at most 10mg KOH/g as a softening agent in a toner for development ofelectrostatic images, which comprises at least a binder resin, acolorant and the softening agent and optionally contains various kindsof additives.

Since this specific organic compound has a good solubility at normaltemperature in a polymerizable monomer, it is easy to be applied to apolymerized toner. This organic compound is preferably a low-softeningpoint substance, particularly preferably a polyfunctional ester compoundhaving a functionality of at least 5. Such an organic compound acts as amodifier such as a softening agent, a parting agent or an anti-offsetagent on a toner. The present invention has been led to completion onthe basis of these findings.

According to the present invention, there is thus provided a toner fordevelopment of electrostatic images, comprising colored particlescontaining at least a binder resin, a colorant and a softening agent,wherein the softening agent is an organic compound having:

(A) a molecular weight of at least 1,000,

(B) a solubility of at least 5 g in 100 g of styrene as measured at 25°C., and

(C) an acid value of at most 10 mg KOH/g.

According to the present invention, there is also provided a process forproducing a toner for development of electrostatic images, comprisingthe step of subjecting a polymerizable monomer composition containing atleast a polymerizable monomer, a colorant and a softening agent tosuspension polymerization in an aqueous dispersion medium containing adispersion stabilizer, said process comprising. using, as the softeningagent, an organic compound having:

(A) a molecular weight of at least 1,000,

(B) a solubility of at least 5 g in 100 g of styrene as measured at 25°C., and

(C) an acid value of at most 10 mg KOH/g.

BEST MODE FOR CARRYING OUT THE INVENTION

1. Softening Agent:

In the present invention, an organic compound having a molecular weightof at least 1,000, a solubility of at least 5 g in 100 g of styrene asmeasured at 25° C. and an acid value of at most 10 mg KOH/g is used as asoftening agent.

The molecular weight of the organic compound used as the softening agentis preferably 1,000 to 1,800, more preferably 1,100 to 1,800,particularly preferably 1,200 to 1,700. If the molecular weight of thesoftening agent is too low, it is difficult to sufficiently lower thefixing temperature of the resulting toner, and the offset resistancethereof also becomes insufficient. If the molecular weight of thesoftening agent is too low, such a softening agent becomes liable tobleed out of the resulting toner during storage of the toner or underhigh-temperature environment in a toner box, and the toner tends tocause a toner filming phenomenon on the surface of a photosensitivemember, or the like in a durability test. When the molecular weight ofthe softening agent falls within the above range, balance among shelfstability, flowability, low-temperature fixing ability and the like ofthe resulting toner becomes good.

The solubility of the organic compound used as the softening agent instyrene must be at least 5 g in terms of a solubility in 100 g ofstyrene as measured at 25° C. (g/100 g of ST; 25° C.). This solubilityis preferably 5 to 25 g, more preferably 8 to 25 g, particularlypreferably 10 to 20 g. If the solubility of the softening agent instyrene is too low, the solubility of a polymerizable monomer composedmainly of styrene is generally lowered. Therefore, the content of thesoftening agent in the resulting toner becomes insufficient, resultingin difficulty in sufficiently lowering the fixing temperature of thetoner. In addition, if the solubility is too low, it is necessary toheat the softening agent to a high temperature for dissolving asufficient amount of the softening agent in the polymerizable monomer.Even if a softening agent poor in the solubility in styrene is dissolvedin the polymerizable monomer at a high temperature, the softening agentis liable to be unevenly dispersed in the resulting polymerized toner.

The acid value of the organic compound used as the softening agent mustbe at most 10 mg KOH/g. The acid value of the softening agent ispreferably 0.01 to 10 mg KOH/g, more preferably 0.01 to 8 mg KOH/g,particularly preferably 0.05 to 5 mg KOH/g. If the acid value of thesoftening agent is too high, an adverse influence is exerted on theformation of droplets of a polymerizable monomer composition in anaqueous dispersion medium, resulting in difficulty in stably formingdroplets even in droplet diameter distribution. Any toner containing asoftening agent high in acid value becomes unstable in charging propertyunder high-temperature and high-humidity environment, resulting indifficulty in achieving sufficient image density. When the acid valuefalls within the above range, a toner sharp in particle diameterdistribution and good in charging property can be provided.

The organic compound used in the present invention and having suchproperties as described above can be considered to function as asoftening agent. However, it is desirable that the organic compound alsohas functions as a parting agent and an anti-offset agent in addition tosuch a function.

Such a softening agent is preferably a low-softening point substanceexhibiting a maximum endothermic peak temperature in a range of 50 to80° C. upon heating thereof in a DSC curve determined by a differentialscanning calorimeter. Such a low-softening point substance can greatlycontribute to the low-temperature fixing ability of the resulting toner.The maximum endothermic peak temperature of the softening agent is 55 to70° C.

As such a softening agent as described above, is particularly preferreda polyfunctional ester compound having a functionality of at least 5. Asexamples of such a polyfunctional ester compound, may be mentionedcondensates of a polyhydric alcohol having a functionality of at least 5and a carboxylic acid. As the polyhydric alcohol, is particularlypreferred dipentaerythritol. As the carboxylic acid, is preferred along-chain carboxylic acid having 10 to 30 carbon atoms. The number ofcarbon atoms in the long-chain carboxylic acid is preferably 13 to 25.As examples of such a long-chain carboxylic acid, may be mentionedmyristic acid, palmitic acid and lauric acid.

In the polyfunctional ester compound used in the present invention, oneor more carboxylic acids may be condensed with the polyhydric alcoholhaving a functionality of at least 5. When at least two carboxylic acidsare used in combination, they are desirably selected in such a mannerthat a difference between the maximum value and the minimum value of thenumber of carbon atoms in said at least two carboxylic acids is at most9, preferably at most 5. Further, the polyfunctional ester compound ispreferably a completely esterified compound, and not a partiallyesterified compound.

Specific examples of the polyfunctional ester compound preferably usedas the softening agent in the present invention includedipentaerythritol hexamyristate, dipentaerythritol hexapalmitate anddipentaerythritol hexalaurate. These polyfunctional ester compounds maybe used either singly or in any combination thereof.

A proportion of the softening agent used is generally 0.1 to 40 parts byweight, preferably 1 to 30 parts by weight, more preferably 5 to 20parts by weight per 100 parts by weight of the binder resin of the toneror the polymerizable monomer forming the binder resin. If the proportionof the softening agent such as the polyfunctional ester compound used istoo low, it is difficult to provide a toner excellent in low-temperaturefixing ability. If the proportion of the softening agent used is toohigh, the offset resistance of the resulting toner is deteriorated, andthe toner filming on the surface of a photosensitive member tends tooccur. In many cases, particularly good results can be yielded when theproportion of the softening agent used is about 8 to 15 parts by weight.

Toner for Development of Electrostatic Images:

The toner for development of electrostatic images according to thepresent invention is not particularly limited by a production processthereof so far as it is composed of colored particles containing atleast a binder resin, a colorant and a specific softening agent.Examples of the binder resin component include (co) polymers of a vinylcompound, such as styrene-acrylic ester copolymers, polyester resins andalicyclic polyolefin resins.

The toner for development of electrostatic images can be obtained by,for example, the grinding process or the polymerization process.Examples of the polymerization process include an emulsionpolymerization process, an aggregation process, a dispersionpolymerization and a suspension polymerization. According to thepolymerization process, toner particles of micron order can be directlyobtained in a relatively narrow particle diameter distribution. Thetoner according to the present invention may also be a toner having acore-shell structure (capsule toner) that a resin coating layer isformed on each surface of the colored particles.

The toner according to the present invention is particularly preferablya polymerized toner obtained by suspension polymerization from theviewpoint of developer properties. The toner of the core-shell structureis preferably obtained by a process comprising forming colored particlesusing as core particles, polymerizing a polymerizable monomer for shellin the presence of the colored particles to form core-shell type polymerparticles in which the colored particles are covered with a polymerlayer formed by the polymerizable monomer for shell.

The volume average particle diameter (dv) of the toner for developmentof electrostatic images (including the toner of the core-shellstructure) according to the present invention is generally 2 to 10 μm,preferably 2 to 9 μm, more preferably 3 to 8 μm, and the particlediameter distribution (dv/dp) represented by a ratio of the volumeaverage particle diameter (dv) to the number average particle diameter(dp) is generally at most 1.6, preferably at most 1.5, more preferablyat most 1.3.

The average thickness of the shell in the toner having the core-shellstructure is generally 0.001 to 1.0 μm, preferably 0.003 to 0.5 μm, morepreferably 0.005 to 0.2 μm. If the thickness of the shell is too great,the fixing ability of the toner tends to be deteriorated. If thethickness is too small, the effect of improving shelf stability of thetoner becomes little.

Production Process of Toner for Development of Electrostatic Image:

A polymerized toner by suspension polymerization may be obtained bysubjecting a polymerizable monomer composition containing at least apolymerizable monomer, a colorant and a softening agent to suspensionpolymerization in an aqueous dispersion medium containing a dispersionstabilizer. A polymer formed by the polymerization of the polymerizablemonomer will become a binder resin. A polymerized toner having thecore-shell structure may be produced in accordance with a spray dryingprocess, interface reaction process, in situ polymerization process,phase separation process or the like. The in situ polymerization processand phase separation process are particularly preferred in thatproduction efficiency is good.

Specifically, the polymerized toner having the core-shell structure canbe obtained by using, as core, colored polymer particles obtained by thepolymerizable monomer composition containing at least the polymerizablemonomer, the colorant and the softening agent to suspensionpolymerization in the aqueous dispersion medium containing thedispersion stabilizer, and subjecting a polymerizable monomer for shellto suspension polymerization in the presence of the core. A polymerlayer formed by polymerization of the polymerizable monomer for shellwill become a resin coating layer. The polymerizable monomer compositionmay contain various kinds of additives such as a crosslinkable monomer,a macromonomer, a molecular weight modifier, a charge control agent, ageneral-purpose parting agent, a lubricant and dispersion aid as needed.

(1) Polymerizable Monomer:

As the polymerizable monomers used in the present invention, ispreferred monovinyl monomers. Specific examples thereof include styrenicmonomers such as styrene, vinyltoluene and α-methylstyrene; acrylic acidand methacrylic acid; derivatives of acrylic acid or methacrylic acid,such as methyl acrylate, ethyl acrylate, propyl acrylate, butylacrylate, 2-ethylhexyl acrylate, dimethylaminoethyl acrylate, methylmethacrylate, ethyl methacrylate, propyl methacrylate, butylmethacrylate, 2-ethylhexyl methacrylate, dimethylaminoethylmethacrylate, acrylonitrile, methacrylonitrile, acrylamide andmethacrylamide; ethylenically unsaturated monoolefins such as ethylene,propylene and butylene; vinyl halides such as vinyl chloride, vinylidenechloride and vinyl fluoride; vinyl esters such as vinyl acetate andvinyl propionate; vinyl ethers such as vinyl methyl ether and vinylethyl ether; vinyl ketones such as vinyl methyl ketone and methylisopropenyl ketone; and nitrogen-containing vinyl compounds such as2-vinylpyridine, 4-vinylpyridine and N-vinylpyrrolidone.

The monovinyl monomers may be used either singly or in any combinationthereof. As the monovinyl monomers, a styrenic monomer and a derivativesof (meth)acrylic acid are preferably used in combination.

(2) Crosslinkable Monomer and Crosslinkable Polymer:

When a crosslinkable monomer and/or a crosslinkable polymer is used inaddition to the polymerizable monomer, the hot offset resistance of theresulting toner can be effectively improved.

The crosslinkable monomer is a monomer having two or more polymerizablecarbon-carbon unsaturated double bonds. Specific examples thereofinclude aromatic divinyl compounds such as divinylbenzene,divinylnaphthalene and derivatives thereof; di-ethylenically unsaturatedcarboxylic acid esters such as ethylene glycol dimethacrylate,diethylene glycol dimethacrylate and 1,4-butanediol diacrylate; otherdivinyl compounds such as N,N-divinylaniline and divinyl ether; andcompounds having three or more vinyl groups, such as trimethylolpropanetriacrylate and trimethylolpropane trimethacrylate.

The crosslinkable polymer is a polymer having two or more polymerizablecarbon-carbon unsaturated double bonds. Specific examples thereofinclude esters of a polymer such as polyethylene or polypropylene, whichhas two or more hydroxyl groups in its molecule, with an unsaturatedcarboxylic acid such as acrylic acid or methacrylic acid.

These crosslinkable monomers and crosslinkable polymers may be usedeither singly or in any combination thereof. The crosslinkable monomerand/or the crosslinkable polymer is used in a proportion of generally atmost 10 parts by weight, preferably 0.01 to 5 parts by weight, morepreferably 0.05 to 2 parts by weight, particularly preferably 0.1 to 1part by weight per 100 parts by weight of the polymerizable monomer.

(3) Macromonomer:

When a macromonomer is used together with the polymerizable monomer, abalance among the shelf stability, offset resistance and low-temperaturefixing ability of the resulting polymerized toner can be improved. Themacromonomer is a relatively long-chain linear molecule having apolymerizable functional group (for example, a unsaturated group such asa carbon-carbon double bond) at its molecular chain terminal. Themacromonomer is preferably an oligomer or polymer having a numberaverage molecular weight of generally 1,000 to 30,000. When amacromonomer having a low number average molecular weight is used, thesurface portions of the resulting toner particles become soft, wherebythe shelf stability of the toner is deteriorated. When a macromonomerhaving a high number average molecular weight is used on the other hand,the flexibility of such a macromonomer is poor, resulting in a tonerdeteriorated in fixing ability.

As specific examples of the macromonomer, may be mentioned polymersobtained by polymerizing styrene, styrene derivatives, methacrylicesters, acrylic esters, acrylonitrile and methacrylonitrile eithersingly or in combination of two or more monomers thereof; macromonomershaving a polysiloxane skeleton (including macromonomers disclosed inJapanese Patent Application Laid-Open No. 203746/1991).

Among the macromonomers, polymers having a higher glass transitiontemperature than that of the binder resin are preferred, with copolymermacromonomers of styrene and a methacrylic ester and/or an acrylicester, and poly(methacrylic ester) macromonomers being particularlypreferred.

When the macromonomer is used, it is used in a proportion of generally0.01 to 10 parts by weight, preferably 0.03 to 5 parts by weight, morepreferably 0.05 to 1 part by weight per 100 parts by weight of thepolymerizable monomer.

(4) Colorant:

As the colorant, may be used any of various kinds of pigments and dyesused in the field of toners, such as carbon black and titanium white. Asexamples of black colorants, may be mentioned dyes and pigments such ascarbon black and Nigrosine Base; and magnetic powders such as cobalt,nickel, triuron tetroxide, manganese iron oxide, zinc iron oxide andnickel iron oxide. When carbon black is used, that having a primaryparticle diameter of 20 to 40 nm is preferably used in that theresulting toner can provide images good in image quality, and the safetyof the toner in environment is enhanced.

As colorants for color toners, may be used yellow colorants, magentacolorants, cyan colorants, etc.

Examples of the yellow colorants include C.I. Pigment Yellow 3, 12, 13,14, 15, 17, 62, 65, 73, 83, 90, 93, 97, 120, 138, 155, 180 and 181;Naphthol Yellow S, Hansa Yellow G, and C.I. Vat Yellow.

Examples of the magenta colorants include azo pigments, fused polycyclicpigments, etc., and specific examples thereof include C.I. Pigment Red48, 57, 58, 60, 63, 64, 68, 81, 83, 87, 88, 89, 90, 112, 114, 122, 123,144, 146, 149, 163, 170, 184, 185, 187, 202, 206, 207, 209 and 25; andC.I. Pigment Violet 19.

Examples of the cyan colorants include copper phthalocyanine compoundsand derivatives thereof, and anthraquinone compounds, and specificexamples thereof include C.I. Pigment Blue 2, 3, 6, 15, 15:1, 15:2,15:3, 15:4, 16, 17and 60; Phthalocyanine Blue, C.I. Vat Blue 6, and C.I.Acid Blue.

The colorants are used in a proportion of generally 0.1 to 50 parts byweight, preferably 1 to 20 parts by weight per 100 parts by weight ofthe binder resin or the polymerizable monomer forming the binder resin.

(5) Molecular Weight Modifier:

As examples of the molecular weight modifier, may be mentionedmercaptans such as t-dodecylmercaptan, n-dodecylmercaptan andn-octylmercaptan; and halogenated hydrocarbons such as carbontetrachloride and carbon tetrabromide. These molecular weight modifiersmay be added before the initiation of the polymerization or in thecourse of the polymerization. The molecular weight modifier is used in aproportion of generally 0.01 to 10 parts by weight, preferably 0.1 to 5parts by weight per 100 parts by weight of the polymerizable monomer.

(6) Lubricant and Dispersion Aid:

A lubricant, such as a fatty acid such as oleic acid or stearic acid, ora fatty acid metal salt composed of a fatty acid and a metal such as Na,K, Ca, Mg or Zn; a dispersion aid such as a silane or titanium couplingagent; and/or the like may also be used with a view toward uniformlydispersing the colorant in the resulting toner particles. Such alubricant or dispersion aid is generally used in a proportion of about1/1,000 to 1/1 based on the weight of the colorant.

(7) Charge Control Agent:

In order to improve the charge properties of the resulting toner,various kinds of charge control agents having positively chargingability or negatively charging ability are preferably contained in thepolymerizable monomer composition. Examples of the charge control agentsinclude metal complexes of organic compounds having a carboxyl group ora nitrogen-containing group, metallized dyes, nigrosine and chargecontrol resins.

More specifically, may be mentioned charge control agents such asBONTRON N-01 (product of Orient Chemical Industries Ltd.), NigrosineBase EX (product of Orient Chemical Industries Ltd.), SPIRON Black TRH(product of Hodogaya Chemical Co., Ltd.), T-77 (product of Hod6gayaChemical Co., Ltd.), BONTRON S-34 (product of Orient Chemical IndustriesLtd-.), BONTRON E-81 (product of Orient Chemical Industries Ltd.),BONTRON E-84 (product of Orient Chemical Industries Ltd.), BONTRON E-89(product of Orient Chemical Industries Ltd.), BONTRON F-21 (product ofOrient Chemical Industries Ltd.), COPY CHRGE NX VP434 (product ofClariant Co.), COPY CHRGENEGVP2036 (product of Clariant Co.), TNS-4-1(product of Hodogaya Chemical Co., Ltd.), TNS-4-2 (product of HodogayaChemical Co., Ltd.), LR-147 (product of The Japan Carlit Co., Ltd.) andCOPY BLUE-PR (product of Hoechst AG); and charge control resins such asquaternary ammonium (salt) group-containing copolymers and sulfonic(salt) group-containing copolymers. The charge control agent is used ina proportion of generally 0.01 to 10 parts by weight, preferably 0.03 to5 parts by weight per 100 parts by weight of the binder resin or thepolymerizable monomer forming the binder resin.

(8) Parting Agent:

Since the polyfunctional ester compound used as the softening agent inthe present invention also acts as a parting agent, the use of any otherparting agent is unnecessary. However, various kinds of parting agentsmay be contained, as needed, for the purpose of, for example, preventingoffset or improving the parting ability of the resulting toner uponfixing by a heated roll.

As examples of the parting agent, may be mentioned low molecular weightpolyolefin waxes such as low molecular weight polyethylene, lowmolecular weight polypropylene and low molecular weight polybutylene;molecular terminal-modified polyolefin waxes such as moleculeterminal-oxidized low molecular weight polypropylene, molecularterminal-modified low molecular weight polypropylene substituted by anepoxy group at its molecular terminal and block polymers of thesecompounds with low molecular weight polyethylene, and moleculeterminal-oxidized low molecular weight polyethylene, molecularterminal-modified low molecular weight polyethylene substituted by anepoxy group at its molecular terminal and block polymers of thesecompounds with low molecular weight polypropylene; vegetable naturalwaxes such as candelilla wax, carnauba wax, rice wax, Japan wax, jojobawax and sasol; petroleum waxes such as paraffin wax, microcrystallinewax and petrolatum, and modified waxes thereof; mineral waxes such asmontan, ceresin and ozokerite; synthetic waxes such as Fischer-Tropschwax; polyfunctional ester compounds such as pentaerythritoltetrastearate, pentaerythritol tetramyristate and pentaerythritoltetrapalmitate; and mixtures thereof.

These parting agents are used in a proportion of generally 0.1 to 20parts by weight, preferably 0.5 to 15 parts by weight, more preferably 1to 5 parts by weight per 100 parts by weight of the binder resin or thepolymerizable monomer forming the binder resin.

(9) Polymerization Initiator:

As the polymerization initiator, is preferably used a radicalpolymerization initiator. As specific examples thereof, may be mentionedpersulfates such as potassium persulfate and ammonium persulfate; azocompounds such as 4,4′-azobis(4-cyanovaleric acid), 2,2′-azobis(2-amidino-propane) dihydrochloride, 2,2′-azobis-2-methyl-N-1,1-bis(hydroxymethyl)-2-hydroxyethylpropionamide,2,2′-azobis(2,4-dimethylvaleronitrile), 2,2′-azobis-isobutyronitrile and1,1′-azobis (1-cyclohexane-carbonitrile); diacyl peroxides such asisobutyryl peroxide, 2,4-di-chlorobenzoyl peroxide and3,5,5′-trimethylhexanoyl peroxide; peroxy dicarbonates such asbis(4-t-butylcyclohexyl)peroxy dicarbonate, di-n-propylperoxydicarbonate, diisopropylperoxy dicarbonate, di-2-ethoxyethylperoxydicarbonate, di(2-ethylethylperoxy) dicarbonate, dimethoxybutylperoxydicarbonate and di(3-methyl-3-methoxybutylperoxy) dicarbonate; andother. peroxides such as (α,α-bis-neodecanoylperoxy)-diisopropylbenzene,cumylperoxy neodecanoate, 1,1′, 3,3′-tetramethylbutylperoxyneodecanoate, 1-cyclohexyl-1-methylethylperoxy neodecanoate,t-hexylperoxy neodecanoate, t-butylperoxy neodecanoate, t-hexylperoxypivalate, t-butylperoxy pivalate, methyl ethyl peroxide, di-t-butylperoxide, acetyl peroxide, dicumyl peroxide, lauroyl peroxide, benzoylperoxide, t-butylperoxy-2-ethyl hexanoate, di-isopropylperoxydicarbonate, di-t-butylperoxy isophthalate and t-butylperoxyisobutyrate. Redox initiators composed of combinations of thesepolymerization initiators with a reducing agent may also be used.

Of these, oil-soluble.radical polymerization initiators soluble in thepolymerizable monomer are preferred. A water-soluble initiator may alsobe used in combination therewith as needed. As the oil-soluble radicalinitiators, oil-soluble radical initiators selected from among organicperoxides whose decomposition temperature giving a half-life period of10 hours are 40 to 80° C., preferably. 45 to 80° C. and whose molecularweights are 300 or lower are preferred, with t-butylperoxy-2-ethylhexanoate and t-butyl peroxyneodecanoate beingparticularly preferred because the resulting polymerized toner barelycauses environmental destruction by volatile components such as odor.

The proportion of the polymerization initiator used is generally 0.1 to20 parts by weight, preferably 0.3 to 15 parts by weight, morepreferably 0.5 to 10 parts by weight per 100 parts by weight of thepolymerizable monomer. If this proportion is too low, the rate ofpolymerization becomes slow. If the proportion is too high, themolecular weight of the resulting polymer becomes low. It is hence notpreferred to use the polymerization initiator in such a too low or highproportion. Although the polymerization initiator may be added into thepolymerizable monomer composition in advance, it may also be added intothe suspension after completion of the step of forming droplets of thepolymerizable monomer composition in the aqueous dispersion medium forthe purpose of, for example, avoiding premature polymerization.

The proportion of the polymerization initiator used is generally 0.001to 3% by weight based on the aqueous dispersion medium. If theproportion of the polymerization initiator used is lower than 0.001% byweight, the rate of polymerization becomes slow. If the proportion ishigher than 3% by weight, the molecular weight of the resulting polymerbecomes low. It is hence not preferred to use the polymerizationinitiator in such a too low or high proportion.

(10) Dispersion Stabilizer:

As examples of the dispersion stabilizer used in the present invention,may be mentioned sulfates such as barium sulfate and calcium sulfate;carbonates such as barium carbonate, calcium carbonate and magnesiumcarbonate; phosphates such as calcium phosphate; metal oxides such asaluminum oxide and titanium oxide; metal hydroxides such as aluminumhydroxide, magnesium hydroxide and ferric hydroxide; water-solublepolymers such as polyvinyl alcohol, methyl cellulose and gelatin; andsurfactants such as anionic surfactants, nonionic surfactants andamphoteric surfactants.

Among these, metallic compounds such as the sulfates, carbonates, metaloxides and metal hydroxides are preferred, with colloid of hardlywater-soluble metallic compounds being more preferred. In particular,colloid of hardly water-soluble metal hydroxides is preferred becausethe particle diameter distribution of the resulting toner particles canbe narrowed, and the brightness or sharpness of an image formed fromsuch a toner is enhanced.

The colloid of the hardly water-soluble metal hydroxide is not limitedby the production process thereof. However, it is preferred to usecolloid of a hardly water-soluble metal hydroxide obtained by adjustingthe pH of an aqueous solution of a water-soluble polyvalent metalliccompound to 7 or higher, in particular, colloid of a hardlywater-soluble metal hydroxide formed by reacting a water-solublepolyvalent metallic compound with an alkali metal hydroxide in anaqueous phase. The colloid of the hardly water-soluble metal hydroxidepreferably has number particle diameter distributions, D₅₀ (50%cumulative value of number particle diameter distribution) of at most0.5 μm and D₉₀ (90% cumulative value of number particle diameterdistribution) of at most 1 μm. If the particle diameter of the colloidis too great, the stability of the polymerization reaction system isbroken, and the shelf stability of the resulting toner is deteriorated.

The dispersion stabilizer is used in a proportion of generally 0.1 to 20parts by weight, preferably 0.3 to 10 parts by weight per 100 parts byweight of the polymerizable monomer. If this proportion of thedispersion stabilizer used is too low, it is difficult to achievesufficient polymerization stability, so that polymer aggregates areliable to form. If the proportion of the dispersion stabilizer used istoo high on the other hand, the particle diameter distribution of theresulting toner particles is widened due to increase in fine particles,and the viscosity of the aqueous solution is increased, so thatpolymerization stability is lowered.

(11) Production Process of Colored Particles:

A polymerized toner can be provided as colored particles composed of apolymer containing a colorant and the like by subjecting a polymerizablemonomer composition containing at least a polymerizable monomer, thecolorant and a specific softening agent to suspension polymerization inan aqueous medium containing a dispersion stabilizer.

More specifically, the polymerizable monomer, colorant, softening agentand other additives (a charge control agent, parting agent, etc.) aremixed, and the resultant mixture is uniformly dispersed by means of abead mill or the like to prepare a polymerizable monomer compositionwhich is an oily liquid mixture. The polymerizable monomer compositionis then poured into the aqueous medium containing the dispersionstabilizer, and the resultant suspension is stirred by a stirrer. Afterthe droplet diameter of droplets of the polymerizable monomercomposition becomes uniform, the polymerization initiator is poured tocause it to migrate into the droplets of the polymerizable monomercomposition.

The droplets of the polymerizable monomer composition are then madefiner by means of a mixing device having high shearing force. In thisdroplet-forming step, droplets having a droplet diameter of generally 2to 10 μm, preferably 2 to 9 μm, more preferably 3 to 8 μm are formed inthe aqueous dispersion medium. If the droplet diameter of the dropletsis too great, toner particles formed become too great, so that theresolution of an image formed with such a toner is deteriorated. A ratioof the volume average droplet diameter to the number average dropletdiameter of the droplets is generally 1 to 3, preferably 1 to 2. If thedroplet diameter distribution of the droplets is too broad, the fixingtemperature of the resulting toner varies, so that inconveniences suchas fogging and filming tend to occur. The droplets preferably have adroplet diameter distribution that at least 30 vol. %, preferably atleast 60 vol. % of the droplets are present within a range of (thevolume average droplet diameter ±1 μm).

After droplets having a droplet diameter almost equal to the particlediameter of a polymerized toner to be formed are formed in theabove-described manner, polymerization is conducted at a temperature ofgenerally 5 to 120° C., preferably 35 to 95° C. In the presentinvention, it is preferred that the aqueous dispersion medium containingthe droplets of the polymerizable monomer composition be prepared in aseparate container or a mixing device, and this dispersion be thencharged into a polymerization reactor to conduct the polymerization.When the droplets are formed in the polymerization reactor, and thesuspension polymerization is conducted as it is, scale is formed in thereactor to easily form a great amount of coarse particles.

Colored polymer particles are formed in such a manner. The coloredparticles formed are recovered and then used as a polymerized toner.

(12) Production Process of Core-shell Type Polymer Particles:

A capsule toner having a core-shell structure may be generally producedin accordance with a spray drying process, interface reaction process,in situ polymerization process, phase separation process or the like.

In the in situ polymerization process preferably adopted in the presentinvention, the colored particles obtained by subjecting thepolymerizable monomer composition containing at least the polymerizablemonomer, the colorant and the specific softening agent to suspensionpolymerization are used as core, and a polymerizable monomer for shellis subjected to suspension polymerization in the presence of the core,thereby forming core-shell type polymer particles.

When a water-soluble polymerization initiator is added upon the additionof the polymerizable monomer for shell to the polymerization reactionsystem, the polymer particles having the core-shell structure are easyto be formed.

As polymerizable monomers for core used in the present invention, thesame polymerizable monomers as described above may be exemplified. Amongthose, a monomer capable of forming a polymer having a glass transitiontemperature of generally at most 60° C., preferably about 40 to 60° C.are preferred as the monomers for core. If the glass transitiontemperature of the polymer component forming the core is too high, thefixing temperature of the resulting toner becomes high. If the glasstransition temperature is too low on the other hand, the shelf stabilityof the toner is deteriorated. In order to adjust the glass transitiontemperature, 2 or more monomers are often used in combination as themonomers for core.

In the present invention, the glass transition temperature (Tg) of apolymer is a calculated value (referred to as calculated Tg) calculatedout according to the kinds and proportions of monomers used inaccordance with the following equation:

100/Tg =W ₁ /T ₁ +W ₂ /T ₂ +W ₃ /T ₃ + . . . W _(n) /T _(n)

wherein

Tg: the glass transition temperature of the copolymer (absolutetemperature),

W₁, W₂, W₃ . . . W_(n): % by weight of the monomers forming thecopolymer composition,

T₁, T₂, T₃ . . . T_(n): glass transition temperature (absolutetemperature) of a homopolymer formed from each of the monomers formingthe copolymer composition.

n: the number of the monomers.

The numbers attached to W and T indicate that such numerical values arethose as to the same monomer.

In the case where the toner according to the present invention is acapsule toner, the volume average particle diameter (dv) of the coreparticles is generally 2 to 10 μm, preferably 2 to 9 μm, more preferably3 to 8 μm. The ratio of the volume average particle diameter (dv) to thenumber average particle diameter (dp) of the core particles is generallyat most 1.7, preferably at most 1.5, more preferably at most 1.3. Thecore particles having such particle diameter and particle diameterdistribution can be obtained by the above-described suspensionpolymerization.

A monomer for shell is added to the core particles thus obtained toconduct polymerization again, whereby a shell layer of the capsule tonercan be formed.

As examples of a specific process for forming the shell, may bementioned a process in which the polymerizable monomer for shell isadded to the reaction system of the polymerization reaction which hasbeen conducted for obtaining the core particles, thereby continuouslyconducting polymerization, and a process in which the core particlesobtained in a separate reaction system are charged, to which thepolymerizable monomer for shell is added, thereby conductingpolymerization stepwise.

The polymerizable monomer for shell may be added to the reaction systemin one lot, or continuously or intermittently by means of a pump such asa plunger pump.

The monomer for shell is that capable of forming a polymer having ahigher glass transition temperature than that of the polymer forming thecore particles. As polymerizable monomers for forming the shell,polymerizable monomers capable of forming a polymer having a glasstransition temperature exceeding 80° C., such as styrene and methylmethacrylate, may be used either singly or in any combination. Herein,the glass transition temperature is a value calculated out in the samemanner as described above.

When the glass transition temperature of a polymer composed of thepolymerizable monomer for the shell is preset so as to become than thatof the polymer composed of the polymerizable monomer for the coreparticles, whereby the fixing temperature of a toner formed can belowered to enhance the shelf stability of the toner. The glasstransition temperature of the polymer formed from the polymerizablemonomer for shell is generally higher than 50° C., but not higher than120° C., preferably higher than 60° C., but not higher than 110° C.,more preferably higher than 80° C., but not higher than 105° C. in orderto improve the shelf stability of the polymerized toner.

A difference in glass transition temperature between the polymer formedfrom the polymerizable monomer for core and the polymer formed from thepolymerizable monomer for shell is generally at least 10° C., preferablyat least 20° C., more preferably at least 30° C.

It is preferable to add a water-soluble radical initiator at the timethe polymerizable monomer for shell is added from the viewpoint of easyprovision of a capsule toner. It is considered that when thewater-soluble radical initiator is added, the water-soluble initiatorenters in the vicinity of each surface of the core particles to whichthe polymerizable monomer for shell has migrated, so that the polymerlayer is easy to be formed on the core particle surface.

As examples of the water-soluble polymerization initiators, may bementioned persulfates such as potassium persulfate and ammoniumpersulfate; azo type initiators such as 4,4′-azobis(4-cyanovalericacid), 2,2′-azobis (2-amidinopropane) bihydrochloride and2,2′-azobis-2-methyl-N-1,1′-bis(hydroxymethyl)-2-hydroxyethylpropionamide; and combinations of anoil-soluble initiator such as cumene peroxide and a redox catalyst. Theamount of the water-soluble polymerization initiator is generally 0.001to 3% by weight based on the aqueous dispersion medium.

A proportion of the polymerizable monomer for core to the polymerizablemonomer for shell to be used is generally 80:20 to 99.9:0.1 in terms ofa weight ratio. If the proportion of the polymerizable monomer for shellis too low, the effect of improving the shelf stability becomes little.If the proportion is too high on the other hand, the improving effect tolower the fixing temperature of the resulting polymerized toner becomeslittle. The thickness of the shell is generally 0.001 to 1.0 μm,preferably 0.003 to 0.5 μm, more preferably 0.005 to 0.2 μm.

(13) Non-magnetic One-component Developer:

When the toner according to the present invention is used as anon-magnetic one-component developer, external additives may be mixed asneeded. As the external additives, may be mentioned inorganic particlesand organic resin particles which act as a flowability-imparting agentand an abrasive.

Examples of the inorganic particles include particles of silicon dioxide(silica), aluminum oxide (alumina), titanium oxide, zinc oxide, tinoxide, barium titanate, strontium titanate, etc. Examples of the organicresin particles include particles of methacrylic ester polymers, acrylicester polymers, styrene-methacrylic ester copolymers and styrene-acrylicester copolymers, and core-shell type particles in which the core iscomposed of a methacrylic ester polymer, and the shell is composed of astyrene polymer.

Among these, the particles of the inorganic oxides are preferred, withsilicon dioxide particles being particularly preferred. The surfaces ofthe inorganic fine particles may be subjected to ahydrophobicity-imparting treatment. Silicon dioxide particles subjectedto the hydrophobicity-imparting treatment are particularly preferred.Two or more of the external additives may be used in combination. Whenthe external additives are used in combination, it is preferable to usetwo kinds of inorganic particles different in average particle diameterfrom each other or inorganic particles and organic resin particles incombination. No particular limitation is imposed on the amount of theexternal additives added. However, it is generally 0.1 to 6 parts byweight per 100 parts by weight of the toner particles. The adhesion ofthe external additives to the toner particles is generally conducted bycharging the toner and external additives into a mixer such as aHenschel mixer to mix them under stirring.

EXAMPLES

The present invention will hereinafter be described more specifically bythe following Examples and Comparative Examples. All designations of“part” or “parts” and “%” as will be used in the following examples meanpart or parts by weight and % by weight unless expressly noted. Physicalproperties and properties in the following Examples and ComparativeExamples were evaluated in accordance with the following respectivemethods.

(1) Solubility in Styrene (g/100 g ST; 25° C.)

The solubility of a softening agent such as a polyfunctional estercompound was determined in terms of an amount (g/100 g ST) of thesoftening agent dissolved in 100 g of styrene at 25° C.

(2) Acid Value (mg KOH/g):

Measured in accordance with JIS K 1557 (1970). About 50 g of a samplewas precisely weighed in a 300-ml beaker, and 128 ml of acetone (80v/v%) were added to this sample. After the sample was dissolved therein,this solution was subjected to potentiometric titration with a 0.1Naqueous solution of NaOH by means of a pH meter to regard a point ofinflection on a titration curve thus obtained as an end point.

The acid value was found in accordance with the following equation:

A=[5.61 ×(B−C)×f]/S

Wherein

A: an acid value (KOH mg/g);

B: an amount (ml) of the 0.1N aqueous solution of sodium hydroxide usedin the titration of the sample;

C: an amount (ml) of the 0.1N aqueous solution of sodium hydroxide usedin a blank test;

f: a factor of the 0.1N aqueous solution of sodium hydroxide; and

S: an amount (g) of the sample used.

(3) Maximum Endothermic Peak Temperature (° C.):

The maximum endothermic peak temperature of a softening agent samplesuch as a polyfunctional ester compound was measured in accordance withASTM D 3418-82. More specifically, a differential scanning calorimeterwas used to heat the sample at a heating rate of 10° C./min, therebymeasuring a temperature exhibiting a maximum endothermic peak on a DSCcurve obtained in the course thereof. When the endothermic peak isbroad, a peak top thereof was judged to be an endothermic peaktemperature. As the differential scanning calorimeter, “SSC5200”manufactured by Seiko Instruments Inc. was used. With respect to eachtoner sample, a maximum endothermic peak temperature attributable to thesoftening agent was measured similarly.

(4) Droplet Diameter (μm) of Polymerizable Monomer Composition:

The volume average droplet diameter (dv), and droplet diameterdistribution, i.e., a ratio (dv/dp) of the volume average dropletdiameter (dv) to the number average droplet diameter (dp) of dropletswere measured by means of a particle diameter distribution meter (SALD2000A Model, manufactured by Shimadzu Corporation). In the measurementby the particle diameter distribution meter, the measurement wasconducted under conditions of a refractive index of 1.55−0.20i, andirradiation time of ultrasonic wave of 5 minutes.

(5) Particle Diameter (μm) of Toner:

The volume average particle diameter (dv), and particle diameterdistribution, i.e., a ratio (dv/dp) of the volume average particlediameter (dv) to the number average particle diameter (dp) of polymerparticles were measured by means of a Multisizer (manufactured byCoulter Co.). The measurement by the Multisizer was conducted under thefollowing conditions:

aperture diameter: 100 μm;

medium: Isotone, concentration: 10%; and

number of particles measured: 50,000 particles.

(6) Thickness of Shell:

Since the thickness of a shell was thin, the thickness of a shell ineach toner sample was calculated out in the following equation:

x=r(1+S/100ρ)^(⅓) r

wherein

r: the radius of core particles before addition of a monomer for shell(a half of the volume average particle diameter of the core particlesfound from measurement by the Multisizer; μm);

x: the thickness (μm) of shell;

s: the number of parts of the monomer for shell added per 100 parts byweight of a monomer for core

ρ:the density (g/cm³) of a polymer forming the shell.

In this measurement, ρ is regarded as 1.0 g/cm³ to calculate out thevalue of x.

(7) Volume Resistivity of Toner:

The volume resistivity of-each toner sample was measured by means of adielectric loss measuring device (TRS-10 Model, trade name; manufacturedby Ando Electric Co., Ltd.) under conditions of a temperature of 30° C.and a frequency of 1 kHz.

(8) Fixing Temperature of Toner:

A commercially available printer (at a rate of 20 paper sheets perminute) of the non-magnetic one-component development system wasmodified in such a manner that the temperature of a fixing roll can bevaried. This modified printer was used to conduct a fixing test. Thefixing test was carried out by varying the temperature of the fixingroll in the modified printer to determine the fixing rate at eachtemperature, thereby finding a relationship between the temperature andthe fixing rate.

The fixing rate was calculated from a ratio of image densities beforeand after a peeling operation using an pressure-sensitive adhesive tape,which was conducted against a black solid-printed area of a test papersheet, on which printing had been made by the modified printer. Morespecifically, assuming that the image density before the peeling of theadhesive tape is “ID_(before)”, and the image density after the peelingof the adhesive tape is “ID_(after)”, the fixing rate can be calculatedout from the following equation:

Fixing/rate (%)=(ID _(after) /ID _(before))×100

The peeling operation of the adhesive tape is a series of operation thata pressure-sensitive adhesive tape (SCOTCH Mending Tape 810-3-18,product of Sumitomo 3M Limited) is applied to a measuring area of thetest paper sheet to cause the tape to adhere to the sheet by pressingthe tape under a fixed pressure, and the adhesive tape is then peeled ata fixed rate in a direction along the paper sheet. The image density wasmeasured by means of a reflection image densitometer manufactured byMcBeth Co.

In this fixing test, a temperature of the fixing roll at which a fixingrate of the toner amounted to 80% was defined as a fixing temperature ofthe toner.

(9) Flowability:

Three kinds of sieves having sieve openings of 150 μm, 75 μm and 45 μm,respectively, are laid on top of another in that order from above, and atoner sample (4 g) to be measured was precisely weighed and put on theuppermost sieve. The three kinds of sieves are then vibrated for 15seconds by means of a powder measuring device (“REOSTAT”, trade name;manufactured by Hosokawa Micron Corporation) under conditions ofvibration intensity of 4. Thereafter, the weight of the toner capturedon each sieve was measured and substituted into its correspondingequation {circumflex over (1)}, {circumflex over (2)} or {circumflexover (3)} shown below, thereby finding the values of a, b and c. Thesevalues were substituted into the equation {circumflex over (4)} tocalculate out the value of flowability. The measurement was conducted 3times on one sample to find an average value thereof.

{circumflex over (1)} a=[(weight (g) of the toner remaining on the sieveof 150 μm)/4 g]×100

{circumflex over (2)} b=[(weight (g) of the toner remaining on the sieveof 75 μpm/)4 g]×100×0.6

{circumflex over (3)} c=[(weight (g) of the toner remaining on the sieveof 45 μm) /4 g]×100×0.2

{circumflex over (4)} Flowability (%)=100−(a+b+c).

(10) Shelf Stability:

Each developer sample was placed in a closed container to seal it, andthe container was then sunk into a constant-temperature water bathcontrolled to 50° C. The container was taken out of theconstant-temperature water bath after 24 hours had elapsed, and thedeveloper contained in the container was transferred to a 42-mesh sieve.At this time, the developer was quietly taken out of the container so asnot to destroy the aggregate structure of the developer in thecontainer, and carefully transferred to the sieve. The sieve wasvibrated for 30 seconds by means of the above powder measuring deviceunder conditions of vibration intensity of 4.5. The weight of thedeveloper remaining on the sieve was then measured to regard it as theweight of the developer aggregated. A proportion ( % by weight) of theweight of the aggregated developer to the weight of the developer firstput into the container was calculated out. The measurement was conducted3 times on one sample to use the average value thereof as an index tothe shelf stability.

(11) Evaluation of Image Quality:

The above-described modified printer was used to continuously conductprinting from the beginning under respective environments of atemperature of 35° C. and a relative humidity of 80% (35° C. ×80% RH;H/H environment) and a temperature of 10° C. and a relative humidity of20% (10° C. ×20% RH; L/L environment)), thereby counting the number ofprinted sheets that continuously retained an image density of 1.3 orhigher as measured by a reflection densitometer (manufactured by McBethCo.) and at an unprinted area, fog of 15% or lower as determined by awhiteness meter (manufactured by Nippon Denshoku K.K.) to evaluate eachdeveloper sample as to environmental dependency.

(12) Durability:

Printing was continuously conducted from the beginning by means of theabove-described modified printer under a room-temperature environment of23° C. in temperature and 50% in RH to count the number of printedsheets that continuously retained an image density of 1.3 or higher asmeasured by a reflection densitometer (manufactured by McBeth Co.) andat an unprinted area, fog of 15% or lower as determined by a whitenessmeter (manufactured by Nippon Denshoku K.K.), thereby evaluating eachdeveloper sample as to the durability of image quality.

Example 11

(1) Preparation of Polymerizable Monomer Composition:

After 100 parts of a polymerizable monomer mixture (Tg of the copolymerobtained by copolymerizing these monomers=55 C.) composed of 80.5 partsof styrene and 19.5 parts of n-butyl acrylate, 6 parts of carbon black(“#25”, trade name; product of Mitsubishi Kagaku Co., Ltd.), 1 part of acharge control agent (“SPIRON Black TRH”, trade name; product ofHodogaya Chemical Co., Ltd.), 0.4 parts of divinylbenzene and 0.5 partsof a polymethacrylic ester macromonomer (“AA6”, trade name; Tg: 94° C.;product of Toagosei Chemical Industry Co., Ltd.) were stirred and mixedby means of an ordinary stirrer, the mixture was uniformly dispersed bymeans of a media type dispersing machine. Ten parts of dipentaerythritolhexamyristate (solubility=at least 10 g; maximum endothermic peaktemperature=63° C.; molecular weight=1514; acid value=0.5 mg KOH/g) wereadded thereto, and mixed and dissolved therein to obtain a polymerizablemonomer composition (liquid mixture). The preparation of allpolymerizable monomer compositions was conducted at room temperature(about 23° C.).

(2) Preparation of Aqueous Dispersion Medium:

An aqueous solution with aqueous solution with 5.8 parts of sodiumhydroxide (alkali metal hydroxide) dissolved in 50 parts ofion-exchanged water was gradually added to an aqueous solution with 9.5parts of magnesium chloride (water-soluble polyvalent metallic salt)dissolved in 250 parts of ion-exchanged water under stirring to preparea dispersion of magnesium hydroxide colloid (colloid of hardlywater-soluble metal hydroxide). The preparation of all dispersions wasconducted at room temperature. The droplet diameter distribution of thecolloid formed was measured by means of an SALD particle diameterdistribution meter (manufactured by Shimadzu Corporation) and found tobe 0.36 μm in terms of D₅₀ (50% cumulative value of number dropletdiameter distribution) and 0.80 μm in terms of D₉₀ (90% cumulative valueof number droplet diameter distribution).

(3) Droplet-forming Step:

The polymerizable monomer composition obtained in the step (1) waspoured into the colloidal dispersion of magnesium hydroxide obtained inthe step (2), and the mixture was stirred until droplets became stable.After 5 parts of t-butyl peroxy-2-ethylhexanoate (“PERBUTYL 0”, tradename, product of Nippon Oil & Fats Co., Ltd.) were added as apolymerization initiator thereto, the resultant dispersion was stirred10 minutes at 15,000 rpm under high shearing force by means of an EbaraMilder (MDN303 Model, manufactured by Ebara Corporation) to formdroplets of the polymerizable monomer composition.

(4) Suspension Polymerization:

A reactor equipped with an agitating blade was charged with the aqueousdispersion containing the droplets of the polymerizable monomercomposition prepared in the step (3) to initiate a polymerizationreaction at 90° C. and continue the reaction for 10 hours. Aftercompletion of the polymerization, the reaction mixture was cooled withwater. While stirring the aqueous dispersion of polymer particlesobtained by the polymerization reaction at room temperature, the pH ofthe system was adjusted to 4.0 or lower with sulfuric acid to conductacid washing (at 25° C. for 10 minutes). After the thus-treateddispersion was filtered to separate water, 500 parts of ion-exchangedwater were newly added to prepare a slurry again to conduct waterwashing. Thereafter, dehydration and water washing were conducted againseveral times repeatedly at room temperature, and solids were separatedby filtration and then dried at 40° C. for a day by a dryer to obtainpolymer particles.

The polymer particles thus obtained had a volume average particlediameter (dv) of 6.1 μm and a ratio of the volume average particlediameter (dv) to the number average particle diameter (dp) of 1.30. Anendothermic peak attributable to dipentaerythritol hexamyristateappeared at 63° C. in DSC measurement.

(5) Preparation of Developer:

To 100 parts of the polymer particles obtained in the step (4), wereadded 0.6 parts of colloidal silica (“RX-200”, trade name; product ofNippon Aerosil Co., Ltd.) subjected to a hydrophobicity-impartingtreatment at room temperature, and they were mixed by means of aHenschel mixer to prepare a non-magnetic one-component developer(hereinafter may be referred to as “toner” merely). The volumeresistivity of the toner thus obtained was measured and found to be 11.3(log Ω·cm).

(6) Properties of Developer:

The fixing temperature of the toner obtained in the step (5) wasmeasured and found to be 140° C. The shelf stability and flowability ofthis toner were very good. The results are shown in Table 1. Besides,evaluation as to image quality revealed that images high in imagedensity, free of fog and irregularity and extremely good in resolutionwere obtained.

Example 2

Polymer particles and a toner were obtained in the same manner as inExample 1 except that the softening agent in Example 1 was changed fromdipentaerythritol hexamyristate to dipentaerythritol hexapalmitate(solubility=at least 5 g; maximum endothermic peak temperature=67° C.;molecular weight=1682; acid value=1.0 mg KOH/g). The results are shownin Table 1. Evaluation as to image quality using the toner thus obtainedrevealed that images high in image density, free of fog and irregularityand extremely good in resolution were obtained.

Example 3

(1) Preparation of Core Particles:

The steps (1) and (2) were conducted in the same. manner as in Example 1except that 5 parts of a yellow pigment (“Toner Yellow HG VP2155”, tradename; product of Clariant Co.) were used as a colorant in place of thecarbon black, and dipentaerythritol hexalaurate (solubility=at least 10g; maximum endothermic peak temperature=56° C.; molecular weight=1346;acid value=0.5 mg KOH/g) was used as the softening agent in place ofdipentaerythritol hexamyristate.

Thereafter, the resultant dispersion was stirred 30 minutes at 15,000rpm under high shearing force by means of an Ebara Milder (MDN303VModel, manufactured by Ebara Corporation) to form droplets of thepolymerizable monomer composition.

The thus-prepared aqueous dispersion containing droplets of thepolymerizable monomer composition was charged into a reactor equippedwith an agitating blade to initiate a polymerization reaction at 60° C.At the time the conversion of the monomer into a polymer reached almost100%, sampling was conducted to measure the particle diameter of coreparticles formed. As a result, the volume average particle diameter (dv)of the core particles was 6.2 μm, and a ratio of the volume averageparticle diameter (dv) to the number average particle diameter (dp) was1.23.

(2) Formation of Shell:

Two parts of methyl methacrylate (calculated Tg of the resultingpolymer=105° C.) and 30 parts of water were subjected to a finelydispersing treatment by an ultrasonic emulsifier at room temperature,thereby obtaining an aqueous dispersion of a polymerizable monomer forshell. The droplet diameter of droplets of the polymerizable monomer forshell was found to be 1.6 μm in terms of D₉₀ as determined by means ofthe SALD particle diameter distribution measuring device.

The polymerizable monomer for shell and 0.2 parts of a water-solubleinitiator (ammonium persulfate, product of Mitsubishi Gas ChemicalCompany, Inc.) were dissolved in 65 parts of distilled water, and thissolution was charged into the reactor to continue the polymerization for4 hours. The reaction was stopped to obtain an aqueous dispersion ofpolymer particles having a pH of 9.5.

While stirring the aqueous dispersion of the core-shell type polymerparticles obtained above at room temperature, the pH of the system wasadjusted to 4.0 or lower with sulfuric acid to conduct acid washing (at25° C. for 10 minutes). After the thus-treated dispersion was filteredto separate water, 500 parts of ion-exchanged water were newly added toprepare a slurry again to conduct water washing. Thereafter, dehydrationand water washing were conducted again several times repeatedly at roomtemperature, and solids were separated by filtration and then dried at45° C. for a day by a dryer to recover polymer particles.

(3) Properties of Core-shell Type Polymer Particles:

The polymer particles thus obtained had a volume average particlediameter (dv) of 6.2 μm and a ratio of the volume average particlediameter (dv) to the number average particle diameter (dn) of 1.24. Thethickness of the shell calculated out from the amount of thepolymerizable monomer for shell used and the particle diameter of thecore particles was 0.02 μm. An endothermic peak appeared at 59° C. inDSC measurement.

(4) Preparation of Developer:

To 100 parts of the polymer particles obtained in the step (3), wereadded 0.6 parts of colloidal silica (“RX-200”, trade name; product ofNippon Aerosil Co., Ltd.) subjected to a hydrophobicity-impartingtreatment at room temperature, and they were mixed by means of aHenschel mixer to prepare a non-magnetic one-component developer(toner). The volume resistivity of the toner thus obtained was measuredand found to be 11.5 (log Ω·cm).

The fixing temperature of the toner obtained above was measured andfound to be 135° C. The shelf stability and flowability of this tonerwere very good. The results are shown in Table 1. Besides, evaluation asto image quality revealed that images high in image density, free of fogand irregularity and extremely good in resolution were obtained.

Comparative Example 1

An experiment was performed in the same manner as in Example 1 exceptthat stearyl stearate (solubility not lower than 5 g, but not higherthan 10 g; maximum endothermic peak temperature=63° C.; molecularweight=536; acid value=4.0 mg KOH/g) was used as the softening agent inplace of dipentaerythritol hexamyristate in Example 1. The shelfstability of the toner was as high as 65%, and this toner was hence notsuitable for practical use. The: durability test was performed. As aresult, filming occurred in the durability test, and fog appeared on atleast 15 sheets among 12,000 sheets. The results are shown in Table 1.

TABLE 1 Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 1 Softening agent: Kind Dipenta-Dipenta- Dipenta- Stearyl erythritol erythritol erythritol stearatehexa- hexa- hexa- myristate palmitate laurate Molecular weight 1514 16821346 536 Solubility ≧10 ≧5 ≧10 5-10 (g/100 g ST; 25° C.) Acid value 0.51.0 0.5 4.0 (mg KOH/g) Endothermic peak 63 67 56 63 temp. (° C.) Amountadded (part) 10 10 10 10 Properties of toner: dv (μm) 6.1 6.2 6.2 6.8dv/dp 1.30 1.28 1.24 1.39 Thickness of shell — — 0.02 — (μm) Volumeresistivity 11.3 11.2 11.5 11.2 (logΩ · cm) Properties of developer:Fixing temperature 140 140 140 145 (° C.) Shelf stability (%) 8 5 orlower 4 65 Flowability 65 62 76 38 Image quality: H/H (sheets) ≧10,000≧10,000 ≧10,000 ≧10,000 L/L (sheets) ≧10,000 ≧10,000 ≧10,000 ≧10,000Durability (sheets) ≧20,000 ≧20,000 ≧20,000    12,000* *Discarded anyfractional sum less than 1,000 sheets.

INDUSTRIAL APPLICABILITY

According to the present invention, there can be provided toners fordevelopment of electrostatic images, which have a low fixingtemperature, can meet energy saving, the speeding-up of printing andcopying, the formation of full-color images, and the like, has excellentshelf stability and flowability and permit forming images high inresolution and good in image quality.

The toners according to the present invention have a low-fixingtemperature and good offset resistance, are excellent in shelf stabilityand can be suitably applied to image forming apparatus for high-speedprinting, and the like.

What is claimed is:
 1. A toner for development of electrostatic images,comprising colored particles containing at least a binder resin, acolorant and a softening agent, wherein the softening agent is anorganic compound having: (A) a molecular weight of 1,000 to 1,800, (B) asolubility of at least 5 g in 100 g of styrene as measured at 25° C.,and (C) an acid value of at most 10 mg KOH/g, and wherein the organiccompound is a polyfunctional ester compound which is a condensate of apolyhydric alcohol having a functionality of at least 5 and a carboxylicacid having 13 to 25 carbon atoms.
 2. The toner for development ofelectrostatic images according to claim 1, wherein the softening agentis an organic compound having: (B1) a solubility of 5 to 25 g in 100 gof styrene as measured at 25° C., and (C1) an acid value of 0.01 to 10mg KOH/g.
 3. The toner for development of electrostatic images accordingto claim 1, wherein the softening agent is a low-softening pointsubstance exhibiting a maximum endothermic peak temperature in a rangeof 50 to 80° C. upon heating thereof in a DSC curve determined by adifferential scanning calorimeter.
 4. The toner for development ofelectrostatic images according to claim 1, wherein the polyhydricalcohol is dipentaerythritol.
 5. The toner for development ofelectrostatic images according to claim 1, wherein the carboxylic acidis at least one carboxylic acid selected from the group consisting ofmyristic acid and palmitic acid.
 6. The toner for development ofelectrostatic images according to claim 1, wherein the polyfunctionalester compound is at least one selected from the group consisting ofdipentaerythritol hexamyristate and dipentaerythritol hexapalmitate. 7.The toner for development of electrostatic images according to claim 1,which has a core-shell structure that colored particles containing atleast the binder resin, the colorant and the softening agent serve as acore, and a polymer layer covering the core is formed.
 8. A process forproducing a toner for development of electrostatic images, comprisingthe step of subjecting apolymerizable monomer composition containing atleast a polymerizable monomer, a colorant and a softening agent tosuspension polymerization in an aqueous dispersion medium containing adispersion stabilizer, said process comprising using, as the softeningagent, an organic compound having: (A) a molecular weight of 1,000 to1,800, (B) a solubility of at least 5 g in 100 g of styrene as measuredat 25° C., and (C) an acid value of at most 10 mg KOH/g, and wherein theorganic compound is a polyfunctional ester compound which is acondensate of a polyhydric alcohol having a functionality of at least 5and a carboxylic acid having 13 to 25 carbon atoms.
 9. The productionprocess according to claim 8, wherein an organic compound having: (B 1)a solubility of 5 to 25 g in 100 g of styrene as measured at 25° C., and(C1) an acid value of 0.01 to 10 mg KOH/g is used as the softeningagent.
 10. The production process according to claim 8, wherein thesoftening agent is a low-softening point substance exhibiting a maximumendothermic peak temperature in a range of 50 to 80° C. upon heatingthereof in a DSC curve determined by a differential scanningcalorimeter.
 11. The production process according to claim 8, whereinthe polyhydric alcohol is dipentaerythritol.
 12. The production processaccording to claim 8, wherein the carboxylic acid is at least onecarboxylic acid selected from the group consisting of myristic acid, andpalmitic acid.
 13. The production process according to claim 8, whereinthe polyfunctional ester compound is at least one selected from thegroup consisting of dipentaerythritol hexamyristate anddipentaerythritol hexapalmitate.
 14. The production process according toclaim 8, wherein said step of subjecting a polymerizable monomercomposition containing at least a polymerizable monomer, a colorant anda softening agent to suspension polymerization is performed to formcolored particles composed of polymer particles containing the colorant,and further comprising the step of polymerizing a polymerizable monomercapable of forming a polymer having a glass transition temperaturehigher than that of the polymer component forming the colored particlesin the presence of the colored particles, thereby forming core-shelltype polymer particles that a polymer layer covering each of the coloredparticles is formed.